The invention refers to an arrangement for illuminating a specimen field in an optical instrument that serves for specimen viewing, as well as a corresponding method.
Uniform and reproducible illumination of a specimen that is to be examined is very important for the functionality of complex optical instruments, for example measurement and inspection systems for the examination of semiconductor wafers. Even slight changes in an illumination device used for this purpose can considerably degrade the performance of the optical instrument.
In particular when high-intensity light sources are used in combination with extremely high-magnification objectives, conventional illumination devices in many cases can no longer meet present-day requirements for high resolution. This applies in particular to demanding inspection and measurement tasks in the UV (ultraviolet) and DUV (deep ultraviolet) regions. The reasons for this are usually inadequate mechanical and thermal stability of the illumination device, and hitherto insufficient alignment capabilities. For example, the changes in illumination caused by the instabilities lower the optical resolution limit for microscopic examinations.
External influences such as vibration and shock thus result in a misalignment between the light source and the illuminating optical system of the illumination device.
As a result of high thermal loading and the effect of strong UV light, changes occur on the internal surfaces of the illumination device housing, in particular on the illuminating optical system and the light source, which negatively affect the intensity of the emitted light over time.
In illumination devices that are used in instruments under clean-room conditions, a sealed housing must be provided in order to prevent contamination of the specimens being examined. This results in the additional problem of a laborious process of replacing the light source, since the housing must be opened in order to make the replacement. In addition, the need to realign the illumination device usually also arises in conjunction with a replacement of the light source. The alignment capabilities available in conventional illumination devices are, however, insufficient in terms of achieving a high illumination quality, or at any rate are very complex with regard to manipulation. If reproducible conditions are to be ensured following a light source replacement, specially trained persons must therefore be used for the purpose.
U.S. Pat. No. 5,925,887 discloses, in conjunction with a projection exposure apparatus, the problem of monitoring the optical exposure parameters when the intention is to work at the limits of technology. In this, the pattern of a mask is aligned with respect to a substrate by means of a projection objective. A device for determining the change in the optical properties of the projection objective is also provided. These changes can result, for example, from heating of the projection objective due to intensive illumination irradiation, but they also depend (inter alia) on the mask, and cause a change in the imaging properties of the projection objective. To compensate for these changes, it is proposed in U.S. Pat. No. 5,925,887 to perform an adjustment of the lens elements of the projection objective that compensates for the deviations. No intervention in the actual illumination device is performed in this context.
It is also already known from U.S. Pat. No. 5,761,336, in order to improve defect detection on semiconductor wafers, to adjust a diaphragm on a microscope in such a way that a maximum detection probability is achieved for specific defect types. Since evaluation of the defects is performed ultimately by an operator, however, the visual effort for defect detection is very great. The risk furthermore exists that changes in the illumination parameters will not be recognized by the operator. Consistent evaluation conditions are thus very difficult to guarantee. Especially in the context of a replacement of the light source, continuity of examination parameters is almost impossible to maintain for examinations at the limit of resolution.
Lastly, U.S. Pat. No. 4,163,150 discloses an arrangement and a method intended to achieve automatic implementation of the Kxc3x6hler illumination principle in a microscope. This is done by measuring an illumination intensity with a photosensor, and using the measured value to control a diaphragm and/or the focal length of a lens arrangement within the illuminating optical system. The purpose of the illumination principle proposed here is to optimize the illumination conditions, in a context of variable image magnification, in terms of achievable resolution and contrast. Changes in the alignment state of the light source are not, however, taken into consideration here.
Proceeding therefrom, it is the object of the invention to create an arrangement for illumination of a specimen field in an optical instrument that makes possible reproducible illumination of the specimen field with highly consistent illumination quality for a specific illumination task.
According to the present invention, this object is achieved with an arrangement of the kind described above which comprises: an illumination device having a light source and an illuminating optical system, the position of the light source and/or of the illuminating optical system being adjustable within the illumination device; a setting device having at least one motorized drive system for positional adjustment of the light source and/or the illuminating optical system; at least one measurement device for sensing parameters of the light generated by the illumination device; and a control device that is configured for the generation of positioning commands for positional adjustment of the light source and/or the illuminating optical system by means of the motorized drive system as a function of the sensed parameters.
By way of the automatic positional setting or alignment of the light source, and optionally also of the illuminating optical system present in the illumination device, the characteristics of the illumination of the specimen field can be adapted to different observation specimens and/or illumination tasks. It is possible to reproduce a specimenxe2x80x94or situationxe2x80x94dependent illumination with little effort in terms of manipulation. Automation of the procedure ensures rapid and easy setting of the corresponding subassemblies, with no need to employ specially trained or instructed operating personnel for the purpose. At the same time, a high illumination quality is achieved so that the best utilization conditions can be consistently achieved with the optical instrument.
In addition, replacement of the light source can also be considerably simplified, since the operator no longer needs to perform settings and alignments in this context. Instead, the entire setting operation after an exchange of the light source can proceed automatically, thereby rapidly re-establishing the performance of the optical instrument after a replacement of the light source.
The arrangement according to the present invention can moreover also be used during operation of the optical instrument, by way of a continuous or quasi-continuous monitoring function, to ensure a consistently high illumination quality. If changes in illumination that negatively affect performance occur during operation of the optical instrument, they can be very quickly corrected. By stipulating an appropriate monitoring regime it is possible, for example in the context of the inspection of semiconductor wafers, to achieve optical defect detection at a consistently high level.
Incandescent lamps, halogen lamps, discharge lamps, or laser light sources are possible, for example, as the light source for the optical instrument. The aforementioned xe2x80x9cilluminating optical systemxe2x80x9d is understood here to be all the optical elements and subassemblies that serve to define the illuminating light. These include, in particular, reflectors, lens elements, lens arrangements, and diaphragms, as well as combinations of such components.
In an advantageous embodiment of the arrangement, a measurement device is arranged on a displaceable stage that also serves to receive and hold a specimen that is to be examined. The consistency of the parameters of the illuminating light can thus be set in direct reference to the actual examination location. This has the advantage that changes between the illumination device and the specimen field can additionally be compensated for by way of the illumination device. In an optical instrument that operates on the incident-light principle, i.e. in which the illuminating light is guided through a portion of the observing optical system of the optical instrument, changes in that optical system can also be taken into account. This also applies analogously to an illumination according to the transmitted-light principle, and correspondingly to components arranged between the illumination device and the specimen field.
A light-sensitive receiving device serving as the measurement device, with which light intensity is measured, is provided for example on the stage. In addition, the distribution of the intensity over a specific region can also be ascertained.
Detectors receiving in one or two dimensions, quadrant detectors, or a CCD matrix or CCD imager can serve, for example, as receiving devices.
If applicable, the light-sensitive receiving device can additionally have placed in front of it an objective, also mounted on the stage and displaceable with the receiving device, so as thereby specifically to improve the receiving conditions.
In principle, it is conceivable to arrange a measurement device in stationary fashion, thereby making possible continuous monitoring of the illumination situation. In measurement devices in which the illumination beam path must be arranged at the location of an observation specimen, however, this results in an impairment of the area usable for the examination. This also applies to measurement devices that are positioned in the observation beam path. Preferably, therefore, a measurement device is configured in such a way that it can be introduced only temporarily into the illumination beam path or observation beam path of the optical instrument. As a result, while the specimen field is fully utilized, rapid checking of the illumination during operation of the optical instrument is nevertheless ensured, i.e. quasi-continuous monitoring is implemented.
In order to check the illumination, it is also possible, for example, to place at the location of an observation specimen a mirror that deflects the incoming light to a measurement device. A separate measurement device can be provided to sense the light reflected by said mirror.
In a particularly advantageous embodiment, what serves as the measurement device is an image receiver that is in any case a component of the optical instrument and is otherwise intended to sense the image of a specimen that is to be examined. In this case a separate measurement device for evaluating the illuminating light is not necessary. By way of the mirror, for example, the total intensity and its distribution in the specimen field, i.e. in an image field plane, can be sensed.
An additional optical system, e.g. a Bertrand lens, that can be temporarily pivoted into the beam path between the specimen and the image receiver can also be provided in this connection. Introduction of the additional optical system furthermore makes it possible to sense the radiation intensity and its distribution in an aperture plane and take it into consideration in setting the light source and, if applicable, also the pertinent illuminating optical system.
A further possibility for checking the illumination consists in the use of test structures that preferably are of similar configuration to the specimens that are to be examined. These test structures are arranged on the stage, and with it can be temporarily shifted into the specimen field. With the test structures it is possible to determine, in particularly quick and reliable fashion, whether a change in the illuminating light has occurred. Evaluation of the illuminating light influenced by the test structures is preferably performed using the actual image receiver of the optical instrument. The positioning signals for any correction of the position of the light source or the illuminating optical system are then determined on the basis of further, more accurate measurements of the illuminating light.
In a further advantageous embodiment of the arrangement, the control device is linked to a database in which various light parameters for the at least one measurement device are stored in specimen-specific and/or task-specific fashion as reference parameters. The reference parameters are obtained, for example, from measurements on reference specimens. Because the reference parameters for specific illuminated specimens or tasks are stored, they are rapidly available so that the illumination device can be switched back and forth between different states. One example of this is the deliberately oblique illumination of a specimen for special measurement devices, for example a partial-pupil illumination or the like. Switching over between bright-field and dark-field illumination is also conceivable.
In principle, it is possible to integrate the illumination device and the setting device into one unit. In an advantageous embodiment, however, the illumination device and setting device are configured as separate modules that can be releasably coupled to one another. This on the one hand makes possible a particularly compact design for the illumination device, but on the other hand ensures that an alignment of the illumination device can be performed quickly and easily when necessary.
The illumination device preferably comprises at least one coupling member, accessible from outside the illumination device, with which the light source and/or illuminating optical system can be adjusted. The setting device is configured in this context as a setting module which can be attached to the illumination device and removed from it, and which comprises a module housing in which the at least one motorized drive system is arranged, and at least one coupling member that is coupled to the drive system and is accessible from outside the module housing, the coupling member on the module housing being configured to transfer a drive motion to a coupling member of the illumination device, and the pertinent drive system being actuable from outside the module housing. This permits particularly simple realignment or also initial alignment after a replacement of the light source.
A setting module that can be attached to the housing of the illumination device, and must remain on the housing of the illumination device only for the alignment time, is used for this purpose. In this fashion, especially in the case of a realignment, it is not necessary to open the housing of the illumination device. Instead, the interfaces created on the exterior of the housing of the illumination device, in the form of one or more coupling members, serve to perform the alignment from outside.
In addition, the setting module can be used as a particularly compact, easily handled alignment tool for multiple illumination devices having identically arranged and embodied interfaces, i.e. coupling members.
The illumination device preferably encompasses the housing in which the light source and the illuminating optical system are arranged, and adjustment devices that are also arranged in the housing and are coupled to the light source and/or the illuminating optical system in such a way as to adjust each of them in at least one coordinate, a portion of the adjustment device being accessible from outside the housing and being configured as a coupling member for the attachable setting module that, as already stated, in turn has a complementary coupling member for transfer of a drive motion.
In its simplest embodiment, an adjustment of the light source in only one single coordinate is brought about with the setting module, for which purpose it is then sufficient to have in the setting module a single drive device that acts on a coupling member thereof.
Preferably, however, the adjustment device of the illumination device has separate linkage elements, connected to the coupling member, for each adjustable coordinate of the light source and/or of the illuminating optical system. It is thereby possible to adjust each of said coordinates separately and independently, so that exact alignment of the light source and/or the reflector is easily achieved. Both translational and rotational adjustment motions can be implemented.
For a light source in the form of an incandescent lamp, halogen lamp, discharge lamp, or LED, for example, three translational and optionally also one rotational degree of freedom are provided for adjustment. If an illuminating optical system having a reflector is used, the latter can, for example, also be adjusted in one translational and two rotational degrees of freedom. For a lens, e.g. a collector lens, three translation degrees of adjustment freedom are usually provided. If the light source and the illuminating optical system are constituted by a laser device, an adjustment of the laser head and/or of any lenses that are present can be performed. It is also possible to align the entire laser device as a unit in several degrees of freedom.
It is also conceivable, however, to provide adjustment capabilities for only some of the aforesaid degrees of freedom or coordinates. In one embodiment of the invention, for example, a lamp is adjustable in three coordinate directions and a reflector is adjustable in only one coordinate direction.
A coupling device for interlocking releasable connection to a coupling device, of complementary configuration, of the setting module is preferably provided on an outer wall of the illumination device housing. This permits temporary attachment of the setting module to the illumination device housing, and moreover creates a centering effect so that reliable connection of the respective coupling members is guaranteed.
The coupling device can be implemented very easily on the illumination device side by way of slot-shaped wall openings that can be produced economically. Movable hooks that can interlock with the slot-shaped wall openings can then be provided, for example, on the setting module side. For decoupling, an unlocking apparatus with which the interlocking engagement can be nullified is provided on the setting module.
The capability of coupling the setting module is, in principle, independent of the configuration of the illumination device housing, in particular of whether the latter is configured as an open or closed housing; what is critical is that the respective coupling members can be brought into positive and/or nonpositive engagement. With regard to the contamination problem mentioned earlier, however, it is advantageous if the housing of the illumination device is completely closed.
In an advantageous embodiment of the setting module, a separate drive device is provided for each coupling member. This results in a high level of setting flexibility. For example, a position correction can be performed individually in a plurality of different coordinates.
The number of coupling members on the setting module preferably corresponds to the number of coupling members on the illumination device. This is advantageous for rapid alignment.
Operation of the setting module and of its drive devices is preferably accomplished by way of relative correction variables for the coordinate direction, for example in the form of increments; as a result, the setting module can be utilized universally for a plurality of different types of housing, and is usable without direct sensing of the actual position of the light source or the illuminating optical system.
Preferably a positioning force limiter or torque limiter is furthermore provided for each drive device. This prevents damage to the adjustment device in the housing of the illumination device. For that purpose, the positioning force limiter or torque limiter is matched appropriately to the adjustment device and to a stop provided for the respective coordinate direction.
If, upon attachment of the setting module to the illumination device, each coupling member of the setting module can be coupled positively or nonpositively to a coupling member of the illumination device, good transfer of the drive motion from the drive devices of the setting module to all the elements in the illumination device that are to be adjusted can easily be achieved. As already stated, a temporary attachment of the setting module to the housing of the illumination device can be realized by way of coupling devices, for example in the manner of a bayonet connector.
In a further advantageous embodiment, a controller is provided that is configured to generate control outputs for each of the drive devices. Also present is an operating console having means for command input with which positioning commands for adjustment of the light source and/or the illuminating optical system can be entered manually.
The controller and the setting module can be configured separately as separate devices that can be temporarily coupled to one another via a connecting conductor. This division into separate units allows a compact design for the setting module, which thus can be attached particularly easily to an illumination device even in confined spaces. As a deviation from this, however, it is also possible to integrate the controller and the setting module into a common housing.
In addition, the operating console and controller can also be configured as separate devices that can be temporarily coupled to one another via a data connection. The data connection can be accomplished by way of a conductor, but alternatively also wirelessly. As a result of the separate arrangement of the operating console, the drive devices in the setting module can be remotely controlled. For example, alignment can be performed directly from a location at which the illumination quality is measurable.
As an alternative to this, however, it is also possible to integrate the operating console and controller into a common housing, or even to integrate the operating console into the optical instrument and connect it permanently thereto.
The operating console can, as a component of the optical instrument, simultaneously also be used to control the latter. An operating console of this kind can also be configured as an external unit that can be temporarily coupled to the optical instrument via a data connection, i.e. via a conductor or also wirelessly. An operating console of this kind can moreover be provided in addition to an operating console integrated into the optical instrument, in which context the external unit, if applicable, needs to possess only limited control capabilities.
In addition to the possibility of embodying the setting device as a unit separable from the optical instrument, one or more measurement devices can also be releasably insertable into the optical instrument so that they can be used with different optical instruments. In combination with a setting module of the kind explained above, the result is thus a setting system that is flexibly usable with a variety of optical instruments. The measurement device is introduced temporarily into the beam path of the illuminating optical system, preferably into an aperture diaphragm plane and/or a field diaphragm plane. Also possible is an arrangement in which the one corresponding measurement device is mounted on an eyepiece of the optical instrument.
In a particularly advantageous embodiment of the arrangement according to the present invention, the control device is configured so that, as a function of the variables sensed with the measurement devices, including the light intensity and/or light intensity distribution measured in the aperture diaphragm plane, the light intensity in the aperture diaphragm plane is maximized by adjusting the position of the light source and/or of the illuminating optical system. By selecting the light intensity in the aperture diaphragm plane as a control loop criterion, a high illumination quality is achieved. Optimum alignment is considered to exist, for example, when the intensity value reaches its maximum. In that case the light source, or its hot spot, filament, or the like, is located in centrally symmetrical fashion in the aperture diaphragm.
The subject matter of the invention furthermore encompasses a method for illumination of a specimen field in an optical instrument utilizing an illumination device having a light source and an illuminating optical system, in which parameters of the light produced by the illumination device are measured; the measured parameters are compared to predefined reference parameters; control outputs are generated as a function of the deviation thereby ascertained; and the control outputs are used to actuate motorized, preferably electromechanical drive systems in order automatically to adjust the position of the light source and/or of the illuminating optical system.
In an advantageous embodiment of the method, a measurement of the parameters of the light is performed during operation of the optical instrument, and upon identification of a deviation, operation of the optical instrument is interrupted for the actuation time of the drive systems. This makes it possible principally to monitor the alignment state of the illumination device during operation.
Preferably a two-stage monitoring regime is used, in which upon identification of a deviation, firstly a further measurement device is activated. Only upon confirmation of the deviation by the further measurement device are the positioning commands for the drive systems generated and the drive systems automatically actuated accordingly. This procedure has the advantage that a fast measurement method can be used for the first step, while greater preference is accorded to the confirmation of accuracy. As already stated above, the measurement can be performed continuously during operation of the optical instrument; preferably, however, it is accomplished at periodic intervals.
Test structures are particularly suitable for rapid checking. This involves measuring, during operation of the optical instrument, the light of a test structure that is positioned in the specimen field or will be positioned for that specific purpose in the specimen field. By evaluation of the light of the test structure, a decision is then made as to whether a more accurate measurement and optionally an alignment of the illumination device are necessary.
If a deviation is identified during operation of the optical instrument, the at least one further measurement is then preferably performed in an aperture diaphragm plane and/or intermediate image plane of the optical instrument.
In addition to monitoring of the illumination, with the method according to the present invention it is furthermore possible to adapt the alignment state of the illumination device to an illuminated specimen or an illumination task. In an advantageous embodiment of the method, for this purpose the predefined parameters are sensed during a calibration operation by measuring real illumination conditions for an illumination task and/or for an illuminated specimen in the optical instrument with its measurement devices, and are stored retrievably in a database. The calibration operation can be performed by way of a measurement with test structures, preferably specimen-like test structures.
Also possible is a procedure in which firstly, in a calibration operation, an optimum setting is obtained using reference specimens whose structure is known. A standardization via a test structure is then performed. This is preferably done by first taking measurements, during the calibration operation, with a reference specimen or several reference specimens at various settings of the illumination device. One of those settings is then defined as the optimum setting. The light of a test structure is measured with the optimum setting of the illumination device. The parameters thereby obtained are then stored as characteristic default parameters (reference parameters).
In the context of resetting the illumination device for a new illumination task and/or a specimen, firstly a basic setting of the illumination device can be made using the test structure and the reference parameters stored for the illumination task and/or the specimen. That is followed by a fine adjustment using the parameters stored for the further measurement devices, and optionally by a further monitoring of the setting in the manner explained above.
Monitoring, or an initial alignment or realignment, is preferably accomplished in such a way that the light intensity in the aperture diaphragm plane is controlled to a maximum, for which purpose the measured parameters, including the light intensity and/or light intensity distribution measured in the aperture diaphragm plane, are sensed as input variables, and control outputs are generated for the motorized drive devices in order to adjust the position of the light source and/or of the illuminating optical system.